Recently, a nonaqueous electrolyte battery such as a lithium-ion secondary battery has been developed as a battery having a high energy density. The nonaqueous electrolyte battery is expected to be used as a power source for vehicles such as hybrid vehicles or electric cars or a large-sized power source for electricity storage. Particularly, for use in vehicles, the nonaqueous electrolyte battery is desired to have other performances such as rapid charge-and-discharge performances and long-term reliability. The nonaqueous electrolyte batteries capable of performing rapid charge-and-discharge have the advantage that a charging time is considerably short. In hybrid vehicles on which the nonaqueous electrolyte batteries capable of performing rapid charge-and-discharge are mounted, power performance can be improved. Moreover, in the hybrid vehicles, regenerative energy can be efficiently recovered the power.
In order to enable rapid charge-and-discharge, it is necessary for electrons and lithium ions to be able to migrate rapidly between the positive electrode and the negative electrode. When a battery using a carbon based material in the negative electrode undergoes repeated rapid charge-and-discharge, dendrite precipitation of metal lithium occurs on the electrode. Dendrites cause internal short circuits, which can lead to heat generation and fires.
In light of this, a battery using a metal composite oxide as a negative electrode active material in place of a carbonaceous material has been developed. Particularly, in a battery using titanium oxide as the negative electrode active material, rapid charge-and-discharge can be performed stably. Such a battery also has a longer life than those using a carbonaceous material.
However, titanium oxide has a higher (nobler) potential relative to metal lithium than that of the carbonaceous material. Further, titanium oxide has a lower capacity per weight. Thus, a battery formed by using the titanium oxide has a problem such that the energy density is low.
For example, the potential of the electrode using titanium oxide is about 1.5 V based on metal lithium and is higher (i.e., nobler) than that of the negative electrode using the carbonaceous material. The potential of titanium oxide is due to the redox reaction between Ti3+ and Ti4+ when lithium is electrochemically inserted and extracted. Therefore, it is limited electrochemically. Further, there is the fact that rapid charge-and-discharge of lithium ions can be stably performed at an electrode potential as high as about 1.5 V. Therefore, it is substantially difficult to drop the potential of the electrode to improve energy density.
As to the capacity of the battery per unit weight, the theoretical capacity of a lithium-titanium composite oxide such as Li4Ti5O12 is about 175 mAh/g. On the other hand, the theoretical capacity of a general graphite-based electrode material is 372 mAh/g. Therefore, the capacity density of titanium oxide is significantly lower than that of the carbon-based negative electrode. This is due to a reduction in substantial capacity because there are only a small number of lithium-insertion sites in the crystal structure of titanium oxide and lithium tends to be stabilized in the structure.
In view of such circumstances, a new electrode material containing Ti and Nb has been examined. Particularly, in a monoclinic Nb—Ti composite oxide represented by TiNb2O7, during the Li-insertion into this composite oxide, charge compensation, in which Ti changes from tetravalence to trivalence and Nb changes pentavalence to trivalence, takes place. As a result, the monoclinic Nb—Ti composite oxide represented by TiNb2O7 exhibits a theoretical capacity of 387 mAh/g, and can exhibit a high capacity, and thus has been a focus of attention.